1. Field of the Invention
The present invention relates to an electrolytic washing apparatus and a steel strip degreasing method using the apparatus. The present invention also relates to a method and apparatus for degreasing steel strips making use of electrolytic washing to remove rolling oil and the like deposited on cold-rolled steel strips.
2. Description of the Related Art
In general, cold-rolled steel strips are degreased by being dipped into an alkali solution or by being subjected to electrolytic washing as well as by washing with brushes (hereinafter, referred to as brush washing) and washing with high pressure water (hereinafter, referred to as high pressure water washing) which are carried out together with the above dipping and electrolytic washing. A dipping type electrolytic washing apparatus and a spray type electrolytic washing apparatus are available as an apparatus for carrying out electrolytic washing. In the dipping type electrolytic washing apparatus, electrolytic washing is carried out using electrodes disposed above and below a steel strip in an electrolytic tank. In the spray type electrolytic washing apparatus, electrolytic washing is carried out by spraying an electrolytic solution onto a steel strip from spray nozzles, which are disposed above and below the steel strip and have electrodes mounted thereon.
FIG. 6 is a schematic view showing a conventional steel strip degreasing apparatus. The arrow in the figure shows the traveling direction of a steel strip. The steel strip 1 is washed with brush rolls 6 after it passes through an alkali dipping tank 3. The steel strip 1 is again subjected to electrolytic washing in an electrolytic washing apparatus 7. Then, after the steel strip 1 is washed with brush rolls 6 again, it passes through a rinse apparatus 9 and a dryer 12, whereby a series of degreasing treatments are carried out.
For example, Japanese Unexamined Patent Application Publication No. 8-174042 discloses a degreasing method and apparatus for carrying out degreasing in a non-contact fashion using dipping type electrolytic washing and high pressure water washing. Japanese Unexamined Patent Application Publication No. 10-237700 discloses another degreasing method and apparatus for carrying out degreasing by spray type electrolytic washing, in which a voltage is imposed on a pair of spray nozzles disposed above and below a steel strip, and by brush washing.
Since a steel strip is generally vibrated also in the thickness direction thereof while it travels, when dipping type electrolytic washing is employed as shown in Japanese Unexamined Patent Application Publication No. 8-174042, the steel strip in travel must be prevented from coming into contact with, or colliding against, the electrodes. For this purpose, the electrodes of an electrolytic washing apparatus must be spaced apart from a steel strip. In particular, the conventional electrolytic washing has a problem in that such a large distance is necessary between the electrodes and the steel strip, that a large amount of electric power is required to carry out electrolytic washing.
Furthermore, the series of steps is complicated in the conventional degreasing treatment, and therefore conventional degreasing apparatus is undesirably large and expensive.
On the other hand, in Japanese Unexamined Patent Application Publication No. 10-237700, the steel strip is prevented from coming into contact with electrodes by virtue of being supported by a high liquid pressure from spray nozzles which also serve as the electrodes; however, this document teaches no technique for properly calculating line conditions to be applied, that is, values to be set to electrodes with respect to line speed: S, thickness of steel strips: TS, width of steel strip: WS, and other parameters such as current: I, length of nozzle: L1, width of nozzle: W, and so on. Accordingly, the steel strip is still insufficiently washed due to inappropriate setting of the electrode parameters. That is, it is difficult to provide sufficient degreasing conditions at all times.
An object of the present invention, which was made to solve the above problems, is to provide a steel strip degreasing method and a steel strip degreasing apparatus capable of performing excellent washing, the degreasing apparatus being arranged such that it can be constructed at a low cost, operating cost can be reduced, that is, electric power required for carrying out electrolytic washing can be reduced, and proper electrolytic conditions can be easily set in accordance with a variety of operating conditions.
The inventors have completed the present invention by finding, in the investigation of a mechanism of electrolytic washing in a degreasing apparatus, that electrolytic washing can be carried out with a small amount of electric power when the relationship between electricity density and current density is maintained within a proper range.
That is, according to the present invention, there is provided a method of degreasing a steel strip which includes the step of carrying out electrolytic washing under the following of electricity density X (C/dm2) and current density Y (A/dm2).
Yxe2x89xa70.01xc3x97Xxe2x88x922 when Xxe2x89xa60.12 
Yxe2x89xa70.7 when 0.12 less than Xxe2x89xa61.0 
It is preferable in the degreasing method that at least one technique selected from brush washing and high pressure water washing be carried out in another stage of said electrolytic washing.
According to the present invention, there is provided an electrolytic washing apparatus which includes electrodes which confront each other across a steel strip located therebetween, have a length L (mm) of 500 to 0.5 mm in the traveling direction of the steel strip and satisfy the condition of the following formula.
500xc3x97(Sxc3x97X)/(60xc3x97Yxc3x97C)xe2x89xa6Lxe2x89xa61100xc3x97(Sxc3x97X)/(60xc3x97Yxc3x97C), 
where
S: linear speed (m/min) of steel strip;
X: electricity density (C/dm2);
Y: current density (A/dm2); and
C: number of electrode pairs.
It is preferable that the electrolytic washing apparatus be a dipping type electrolytic washing apparatus having electrodes whose length L is 500 to 10 mm or a spray type electrolytic washing apparatus having electrodes whose length L is 50 to 0.5 mm.
Further, according to the present invention, there is provided a steel strip degreasing apparatus including any of the above electrolytic washing apparatuses.
It is preferable that the steel strip degreasing apparatus includes at least one section selected from a brush washing apparatus and a high pressure water washing apparatus, in addition to any of the electrolytic washing apparatuses.
Furthermore, in the present invention, there is provided a method in which any of the above electrolytic washing apparatuses and any of the above steel strip degreasing apparatuses are used and any of the above steel strip degreasing methods is carried out.